A non-vitreous body can have a very poor bond with the glaze

Notes

This glaze is on very thick. That gives it the power to impose its thermal expansion (which is different that the body) to the point where it literally flakes off. The problem is worsened when the glaze and body lack fluxes, that means they do not interact, no glassy interface is formed.

Links

Glossary

InterfaceIn ceramics, the zone of adherence between glaze to the underlying body is called the clay-glaze interface. The integrity of this interface is important to strength and functionality.